In a CDMA (Code Division Multiple Access) mobile communication system, since signals of different users are correlated to some extent, MAI (Multiple Address Interference) has become a main interference in a broadband CDMA system. A traditional signal separation method in the CDMA system is to regard the MAI as the noise and the separation of signals of different users as independent, i.e. the single user detection technology. According to this detection method, a SNR will be seriously deteriorated and the system capacity will be limited. In fact, a lot of information for improving accuracy of signal separation is included in the MAI, such as a known user channel code, channel estimation of each user, which is used in a multi-user detection technology to perform the joint-detection. According to different multiple access interference processing methods, the multi-user detection technology can be interference counteracting or joint-detection. A basic idea of the interference counteracting is decision feedback, which firstly decides part data from all the received signals and then reconstructs signals corresponding to the data according to the data and user spreading code and then subtracts the reconstructed signals from all the received signals and thus in a cycle and iteration way. The joint-detection technology refers to a signal separation technology for separating all users' signals in one step by fully making use of the MAI.
In the CDMA system, the joint-detection technology may be adopted to eliminate the MAI and intersymbol interference of signals received in the home cell. TD-SCDMA (Time Division Synchronous Code Division Multiple Access) system is a time-slotted CDMA system which performs the joint-detection technology on multi-user's signals in the home cell.
In the TD-SCDMA system, a burst signal structure in a normal slot is shown in FIG. 1, wherein a midamble (also called channel estimation code) in the middle of the burst signal is used to perform the channel estimation and data blocks at the two sides are used to transmit service data.
The single cell joint-detection method uses the structure information (including spreading code and channel response) of signals transmitted by all the users in the home cell and regards interference signals from other cells as Gauss white noise. Therefore, the single cell joint-detection method is capable in suppressing the intersymbol interference and the MAI in the home cell and can greatly improve the system performance for a single cell or differ-frequency CDMA system.
However, in a case of universal-frequency networking, there is a strong interference between neighboring cells with the same frequency (hereinafter referred to as neighboring universal-frequency cells) and this kind of interference influence very much on the system performance. Especially when multiple code channels are working at a boundary between the neighboring universal-frequency cells, the universal-frequency interference is the most important interference. And at this time, the single cell joint-detection method is incapable of suppressing the neighboring universal-frequency cell interference.
If the structure information of neighboring universal-frequency cells is used to expand the joint-detection method from a single cell to the universal-frequency multiple cells, the performance of the time-slotted CDMA system in universal-frequency networking will be greatly improved. The Chinese patent application 200410080196.6 proposes “a method for performing multi-user joint-detection in a neighboring cell in a TDD-CDMA system” in which channel estimation results of neighboring cells are obtained according to a multiple code set channel estimation method and code channels of neighboring cells are grouped and then the joint-detection is performed according to the channel estimation result of neighboring cells and the result of code channel grouping. If this joint-detection method is applied in the time-slotted CDMA system with multiple universal-frequency cells, the MAI between neighboring universal-frequency cells is greatly suppressed and the performance of the time-slotted CDMA system when working in neighboring universal-frequency cells is improved. This technical solution is applicable to cases when user signals in different cells are synchronous or near synchronous or with little comparative delay.
Generally, there is a comparative delay between signals of universal-frequency cells. If only the interference from the cell nearest cell is taken into account, the comparative delay is small so if the channel response window of each user in different cells is obtained in its own cell, the main path can be guaranteed to be in the window and the influence of the comparative delay can be neglected. However, in a large-scale networking, the universal-frequency interference from peripheral cells should be taken into account and in a layered networking, the universal-frequency interference between macro cells and micro cells should be taken into account. For example, in a schematic diagram showing relations between main paths in the channel estimation results of users in neighboring cells and home cell shown in FIG. 2, neighboring cell 1 has a comparatively small delay □T1 with the home cell and if the channel estimation result of each user in the cell is intercepted according to the present method it can be guaranteed that the main path is within the result; while neighboring cell 2 has a comparatively large delay □T2 with the home cell and if the channel estimation result of each user in the cell is still intercepted according to the present method the main path of the user may be intercepted by other users in the cell so that the universal-frequency interference cannot be properly eliminated and the system performance deteriorates.